1. Field of Invention
The present invention relates to a proton precession magnetometer sensor which senses strength of a magnetic field by measuring frequency of current induced in a coil due to a break of strong current after flowing the strong current in the coil.
2. Description of Related Art
In geophysical prospecting, a magnetic survey is a method that provides fundamental and important information for a reconnaissance survey and a detail survey. To this end, a 3-axis fluxgate magnetometer, a proton precession magnetometer, an Overhauser effect magnetometer, an optical-pumping magnetometer, etc. are employed as a magnetometer for surface and aerial surveys.
The fluxgate magnetometer is capable of measuring vector components, but its temperature drift characteristic or orthogonality is not precise. The Overhauser effect magnetometer is experimentally vulnerable to ambient noise such as 60 Hz power line or the like. The optical-pumping magnetometer is inconvenient in that it needs direct-tuning of a manufacturer again for replacement since a current technology level cannot strictly predict a lifespan of an internal lamp.
Meanwhile, the proton precession magnetometer measures a total magnetic field on the basis of an accurate frequency count, so that a precise magnetic field can be obtained as long as a reference oscillation frequency has a good temperature characteristic. Together with an aeromagnetic survey, the proton precession magnetometer has been widely used in the world.
However, such methods for measuring a magnetic field using the principle of proton precession has a limit that a low-sensitivity angle (i.e., a dead band) exists at a certain direction where the measurement is not impossible. The principle of the proton precession is as follows.
An atomic nucleus, i.e., a proton has a spin, that is, properties of the quantum mechanics. The spin of the proton has a magnetic quantum number depending on quantized angular momentum and has a certain orientation. If the proton does not have a total spin quantum number of “0” like 1H(proton) or 13C, it has magnetic susceptibility. The proton has a spin of ½ and its possible spin states are m=±½. Like this, different states having the same energy level are called superposition. These two different states have the same population under thermal equilibrium, but the superposition is broken when a strong magnetic field is applied to the proton. Thus, the magnetic moment of the proton is aligned with the direction of the applied magnetic field. At this time, if the applied magnetic field is removed, the earth's magnetic field has an effect on the magnetic moment of the proton and thus the proton precesses when the earth's magnetic field acts in a nonparallel direction to the magnetic moment of the proton as given in the following expression 1. Here, a component of the earth's magnetic field, which is orthogonal to the magnetic moment of the proton, causes the precession. In conclusion, the earth's magnetic field should not be parallel with the magnetic moment of the proton as shown in the following expression 1 to cause the precession (for example, there is no precession when an angle between the magnetic moment of the proton and the earth's magnetic field is “0” or “180” degrees).dir({right arrow over (Hear)})dir({right arrow over (Hexc)})  [Expression 1]
The precession is a phenomenon that a rotation axis of a spinning body turns around a stationary axis, which is caused as moment of weak external force acts orthogonally. As an example of the precession, there is the earth's rotation axis, a rotation axis of a satellite, etc. Meanwhile, the atomic nucleus, i.e., the proton has the magnetic moment depending on the spin quantum number, and precesses with regard to a component of a weak external magnetic field (e.g., the earth's magnetic field) orthogonal to the magnetic moment if the external magnetic field acts in a different direction to the magnetic moment. Here, the frequency (fprec) of the precession is in proportion to the strength (Hear) of the external magnetic field.
The proton precession magnetometer uses the foregoing principle, in which frequency (this is equal to the frequency (fprec) of the precession) of current induced in a solenoid coil by flowing and then breaking current in the solenoid coil is measured to calculate the strength of the external magnetic field. That is, if the strong current flows in the solenoid coil, a magnetic field is created inside the solenoid coil in a direction of penetrating the solenoid coil. This magnetic field makes the magnetic moment of the proton inside the solenoid coil be aligned with the direction of the applied magnetic field (i.e., the direction of penetrating the solenoid coil). At this time, if the current is broken, the magnetic moment of the proton is affected by the earth's magnetic field, so that the proton can precess.
As shown in the expression 1, the precession arises when the magnetic moment of the proton and the earth's magnetic field are not parallel with each other. On the other hand, as shown in the following expression 2, if the magnetic moment of the proton and the earth's magnetic field are parallel with each other, for example, if the earth's magnetic field is in parallel with a direction of penetrating a solenoid coil of a proton precession magnetometer sensor, the precession is not caused since there is no external force that moves the axis of the magnetic moment of the proton, and therefore it is impossible to measure the strength of the earth's magnetic field.dir({right arrow over (Hear)})∥dir({right arrow over (Hexc)})  [Expression 2]
Substantially, induced voltage has the highest amplitude in a direction where the magnetic moment of the proton is orthogonal to the earth's magnetic field. On the other hand, the induced voltage decreases as the two magnetic fields get more parallel with each other, and therefore it is difficult to measure the frequency. In other words, a conventional proton precession magnetometer has a limit that a low-sensitivity angle (i.e., a dead band) exists at a certain direction where the measurement is not impossible. Accordingly, the conventional proton precession magnetometer needs a position adjustment of a sensor to make the magnetic field inside the solenoid coil be orthogonal to the earth's magnetic field when measuring the earth's magnetic field.